1. Field of the Invention
The present invention relates to a photoelectric conversion device. In particular, the present invention relates to a MOS photoelectric conversion device including a MOS transistor.
2. Description of the Related Art
In recent years, demands for photoelectric conversion devices as image pickup devices for use in two-dimensional image input apparatuses, such as digital still cameras and camcorders, and for use in one-dimensional image readers, such as facsimiles and scanners, have been rapidly increasing.
Charge-coupled devices (CCDs) and MOS photoelectric conversion devices are used as photoelectric conversion devices.
In photoelectric conversion devices, it is necessary to reduce noise generated in photoelectric conversion regions. An example of such noise is noise caused by hot carriers generated in MOS transistors disposed in photoelectric conversion regions. The term “hot carrier” refers to a carrier generated by subjecting a p-n junction constituted by a drain region and a channel end to a strong electric field generated by applying a voltage to a gate of a MOS transistor. In devices such as photoelectric conversion devices that handle weak signals, noise generated by hot carriers, in particular, may lead to a problem.
As an example of a method for reducing noise, Japanese Patent Laid-Open No. 11-284167 (Patent Document 1) and Japanese Patent Laid-Open No. 2000-012822 (Patent Document 2) each disclose a MOS transistor that has a lightly doped drain (LDD) structure and that is disposed in a photoelectric conversion region. This structure reduces the strength of an electric field applied to a drain and a channel formed below a gate and thus can reduce the effect of hot carriers.
In addition, Patent Document 2 discloses a process for producing a structure including a MOS transistor that has the LDD structure and that is disposed in a photoelectric conversion region. The process will be briefly described with reference to FIG. 2 of Patent Document 2. A light-receiving portion and a detecting portion described below serve as a source and a drain, respectively, of a transfer transistor.
A region to be formed into a light-receiving portion is subjected to ion implantation. To form a lightly doped semiconductor region in a detecting portion, ion implantation is performed. A silicon nitride film functioning as an anti-reflection film for the light-receiving portion is formed so as to cover the light-receiving portion, a gate electrode, and the detecting portion. The silicon nitride film is patterned on the gate electrode to form a side wall on the drain side of the gate electrode. A heavily doped semiconductor region is formed with the side wall as a mask to form a photoelectric conversion device.
In recent years, photoelectric conversion devices have been required to have higher pixel densities and larger numbers of pixels while photoelectric conversion properties, such as sensitivity and a dynamic range, have been maintained or improved. Reducing the driving voltage of a photoelectric conversion region and miniaturizing a region other than the photo-receiving portion while a reduction in the area of the photo-receiving portion is inhibited are effective in fabricating such photoelectric conversion devices.
However, the miniaturization of the MOS transistor for reading a signal in response to a signal charge of a photoelectric conversion element disposed in the photoelectric conversion region may degrade the reliability of transistor properties.
In the above-described process, the width of a side spacer is equal to that of a peripheral circuit region. Thus, when an electric field-reducing structure optimized for the peripheral circuit region is designed, a reduction in the electric field strength in the photoelectric conversion region may be insufficient. In this case, hot carriers degrade the reliability of the MOS transistor. Thus, to ensure reliability, the MOS transistor needs to have a larger gate length. This results in a disadvantage to miniaturization.
Furthermore, in the above-described process, the anti-reflection film in the photoelectric conversion region is subjected to etching. Etching causes damage (mainly plasma damage) to the photoelectric conversion region. This increases a dark current flowing through a photodiode.
To overcome at least one of the foregoing problems, the present invention provides a photoelectric conversion device having improved properties without an increase in the number of production steps.